The invention herein relates to construction elements of the kind comprising two plates of nailable material and at least one web which keeps the plates in spaced parallel positions. The material of the plates may be solid wood, for example sawed boards, or plywood, chipboard, wood fibre board or the like. Elements of the said kind may be made with one, two or more webs and also be of various design in other respects, for example as I-, H- or box beams or as larger panel elements adapted for various purposes. In particular such elements are used in the form of so-called "stressed-skin" elements as load-supporting floor and roof elements, mostly of closed, box-like shape and filled with insulating material, in which case the webs are most frequently constituted by wooden joists to which the plates are nailed and/or glued so as to form upper and lower beams in the structure.
However, it is also known to make elements of the kind mentioned above with webs in the form of elongated sinuous metal sheets, preferably steel sheets, which along the lateral edges are formed with pointed teeth which are pressed into the plate members. A structure of this kind is disclosed in the German published patent specification No. 1,004,790 (Hess) of 1954. In the form shown and described in that publication this concept, however, is of little practical interest, the attachment of the web sheet in the plate affording little resistance to separating forces and, for such reason, the plates must be held together by bolts passing therethrough.
In the U.S. Pat. No. 3,538,668 (Anderson) it has been proposed to increase the pull-out force, i.e. the resistance of the teeth to being pulled out of the plates, by bending the teeth alternately to opposite sides like the teeth of a saw blade, so that, when pressed in, they will be further deflected so as to be locked in the wood plate. At the same time it is proposed to reinforce the teeth locally by giving them an angular cross-sectional shape. However, in this case the teeth are subjected to considerable bending during assembly, so that they must be made very short in order not to collapse. Especially with hard fibre board and chipboard of qualities at present manufactured as floor boards, it has turned out to be very difficult to achieve a satisfactory connection in this manner with the use of economically justifiable steel qualities and thicknesses.
There are also known fastener elements for joining together two pieces of wood lying one on top of the other. One such fastener element, which is disclosed in U.S. Pat. No. 3,458,518 (Heise) consists of a strip of sheet metal which along opposite edges is formed with pointed teeth that are to be pressed into the opposite wooden members, thus forming a hidden connection. The teeth disclosed in this patent are long and slender, about 3 to 4 times as long as they are wide, and are stiffened by one single bend along the tooth axis, so that the teeth aquires a shallow V- or U-shaped section.
In U.S. Pat. No. 3,736,718 (Sylvan) is disclosed a stud or a post member in the form of a rod-like element of sheet metal, with a Z-shaped cross section. At the ends of the rod are formed pointed teeth, also stiffened by one bend along the tooth axis and thus having a shallow V- or U-shaped section. The rod is designed to function as a post member or a stiffening rib in a framework for a wall panel, and is capable of taking axial compression and some transverse bending, but is not a shear transferring member. The pointed teeth have negligible stiffness compared to the stud member proper.
For the reasons given in detail below, none of the teachings of the above mentioned patents forms a basis for the design of a web member in the form of a continuous sheet metal strip, that meets the technical and economical requirements essential for a commercial exploitation.